The present invention relates to a method of manufacturing calcined aluminum fluoride, AlF.sub.3, from aluminum fluoride hydrate, especially from aluminum fluoride trihydrate, AlF.sub.3.3H.sub.2 O, by drying and calcining.
In the crystallization of aluminum fluoride, AlF.sub.3, from aqueous solutions, hydrates of the aluminum fluoride are obtained, among others especially aluminum fluoride trihydrate, AlF.sub.3.3H.sub.2 O. The dehydration of the hydrate presents problems, because during the heating to the calcining temperature, usually of 500.degree.-600.degree. C., in contact with the separated steam, a hydrolysis reaction takes place, schematically roughly according to the equation: EQU 2AlF.sub.3 +3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +6HF
If aluminum fluoride hydrate is calcined in a single step in a directly heated lined rotary furnace in countercurrent, then the result is a calcined product with an AlF.sub.3 content of only 86 up to a maximum of 94%. The remainder is substantially Al.sub.2 O.sub.3, produced by the hydrolysis reaction referred to above, and SiO.sub.2 which, during crystallization of aluminum fluoride hydrate, is precipitated with it.
If aluminum fluoride is used in aluminum fusion electrolysis directly, or for the production of cryolite, Na.sub.3 AlF.sub.6, for the same purpose, then a content of SiO.sub.2 in the aluminum fluoride of around 0.3% by weight is acceptable, but however the lowest possible SiO.sub.2 contents are desired, because SiO.sub.2 disturbs the progress of the electrolysis and produces a raw aluminum which cannot be employed for all purposes. A higher content of Al.sub.2 O.sub.3 in aluminum fluoride would, in itself, not have a disturbing effect during the electrolysis, because alumina is used as well. A smallest possible Al.sub.2 O.sub.3 content in the aluminum fluoride is aimed at on economic grounds. An attempt is made to keep as high as possible the efficiency of use of the fluosilicic acid, H.sub.2 SiF.sub.6, supplied for the manufacture of aluminum fluoride hydrate, which naturally is not the case with the occurrence of hydrolysis by the separation of hydrogen fluoride.
A number of methods are known which extensively exclude the hydrolysis reaction, so that products are obtained the AlF.sub.3 content of which is .gtoreq.96% by weight, but which however still contain SiO.sub.2 in the range of around 0.3% by weight.
The hydrolysis reaction can be extensively prevented by step-wise dehydration of the hydrate: In the 1st step the aluminum fluoride hydrate, generally aluminum fluoride trihydrate, AlF.sub.3.3H.sub.2 O, is dried in the usual way and partly dehydrated; in the 2nd step complete dehydration and calcining occurs.
It is technically simple and very economic to carry out the 2nd step by means of a rotary furnace, especially an indirectly heated rotary furnace according to U.S. Pat. No. 4,248,849. Accordingly, product which, according to U.S. Pat. No. 3,991,171 has been calcined in a disc drier from aluminum fluoride trihydrate, AlF.sub.3.3H.sub.2 O, to aluminum fluoride semihydrate, AlF.sub.3.0.5H.sub.2 O, is calcined in an indirectly heated rotary furnace to anhydrous aluminum fluoride, AlF.sub.3.
A hitherto unsolved problem during calcining, especially with employment of an indirectly heated rotary furnace, are the formations of crusts or adhesions of aluminum fluoride on the drum wall. These build up during the calcining and hinder continuous operation. In addition, the heat transfer to the material to be calcined in the rotary furnace is strongly hindered, which greatly reduces in consequence the output of the furnace. It is very difficult to remove the adhesions again from the furnace wall. For this purpose, the furnace must be started and stopped. This is associated with significant losses of production.
A two-fold objective therefore underlay the present invention, firstly to find a continuous method for calcining of aluminum fluoride hydrate, especially aluminum fluoride trihydrate, AlF.sub.3.3H.sub.2 O, which does not have the above-mentioned disadvantages, with the employment of a rotary furnace, especially an indirectly heated rotary furnace, and secondly, to find a method of manufacture of aluminum fluoride, AlF.sub.3, which has an AlF.sub.3 content of .gtoreq.96% by weight with a low SiO.sub.2 content, preferably of .ltoreq.0.1% by weight. In addition, the product should also be free of dust, because in charging during the electrolysis not insignificant quantities are eddied into the air by the flow of heat and by the draught in the furnace hall, which makes working at the furnaces more difficult.